The invention described herein relates to nuclear reactors and more particularly to a system for transferring fuel assemblies between a fuel handling building and the containment area which encloses the reactor.
The fuel for large nuclear reactors of the type used for generating electrical power, is contained in long, small diameter fuel rods or elements ranging in length from 12 to about 20 feet. Typically, about 225-400 fuel elements are disposed in a predetermined pattern in a fuel assembly with spaces provided between the fuel elements for vertically adjustable control rods. Although the number of fuel assemblies may vary from reactor to reactor, a perspective on reactor size may be appreciated from the fact that a 1100 megawatt reactor will contain 200 fuel assemblies. When these fuel assemblies are set in position and the reactor placed in operation, the fission process generates heat and in so doing, consumes the fuel thus requiring removal and replacement of old fuel assemblies with new assemblies containing fresh fuel. Since the metallic rods and other supporting structures in each assembly becomes radioactive, the operation which effects transfer of old for new assemblies must be carried out completely underwater. As with usual reactor constructions, the reactor head and associated components are removed and the reactor containment flooded to a level sufficient to permit keeping the assembly submerged as the assembly is lifted vertically from the reactor core and transferred to a shipping container.
According to past practices, each fuel assembly is removed from the reactor to a transfer area wherein the assembly is pivoted at one end and lowered for movement horizontally through a canal to a spent fuel pit. To protect against damage to fuel elements and release of radioactive gases in the event of cable breakage or other accident, an elaborate shock absorber arrangement is used to decelerate the container as it falls. The lifting and shock absorber structure needed for this purpose occupies the working area space and obstructs the movement of equipment used in the transfer process. Also, each assembly is moved to the spent fuel area by a transfer car powered by an underwater air motor, sprocket and chain arrangement connected to the car and the housing. Should the sprocket and chain break or the motor become inoperative, the canal thereafter must be drained of radioactive water to effect repair to the equipment.
Underwater limit switches also used for signaling, controlling and interlocking various operations in the system may become inoperable, and as bypass switches are then used to carry out the transfer operation, the protection afforded by the limit switches is lost and the possibility then exists .mu. fuel elements inadvertently could be damaged because of lack of adequate protective systems.
The above discussion illustrates that the disadvantages inherent in prior systems suggests the need exists for an improved fuel transfer system which will be less costly but more efficient to provide the reliability needed for nuclear reactor operations.